This invention relates in general to an apparatus for the non-contact optical detection of an edge of an object, for example, the front and/or back edge of a paper sheet on a transparent transporting belt of a printing machine.
The detection of the entry of the edge of an object into an area to be monitored is a common task for photoelectric barriers. The switching precision (in which the object recognized as present or not present) depends essentially on the shape and size of the light beam used for the detection, and on the switching thresholds in the evaluation of the receiver signal of the photoelectric barrier.
The object of the invention is to detect the entry of an edge of an object (2), located on a sufficiently transparent transporting belt (1), for example, a paper sheet, into a theoretical position with a high degree of precision. It is thereby essential that the edge to be detected is reliably distinguished from other features of the transporting device, such as markings and scratches on the transporting belt (1) or connecting seams of the beltxe2x80x94that is, the sensor apparatus should not recognize these features as the edge of the object to be detected (2). On the other hand, the detection of these additional features provides essential information for the operation of the transporting device; for this reason, their appearance should also be reliably identified and signaled via separate switching outlets of the photoelectric barrier apparatus (3,4). An essential boundary condition is to be found in the fact that both the adhesive seams and other connecting seams of the transporting belt (1), as well as the markings, primarily appear transverse to the direction in which the belt is moving.
To attain this goal, a method and apparatus with the features described by the embodiments and developments of the invention is described below.
In accordance with the invention, the reliable identification of the edge of the object is made possible by an apparatus consisting of severalxe2x80x94in the embodiment presented, for example, threexe2x80x94separate, single-way photoelectric barriers, below referred to as measurement sites that, on the one hand, have a specific spatial apparatus and specially shaped light beam cross sections whose signals, on the other hand, are linked logically in an evaluation unit in such a way that the detection of the different features is made possible. This evaluation unit has separate outlets for the features to be detectedxe2x80x94the edge of the object, connecting or adhesive seam, markings, and scratches.
The high precision of the switching point required for the detection of the edge of the sheet is attained by a suitably shaped light beam cross section of one of the measurement sites (5). This light beam has as short as possible an extensionxe2x80x94for example, only a few micrometersxe2x80x94in the direction of movement of the belt (F). Thus, a high dynamics of the signal is brought about over a short travel distance of the transporting belt (1), so a very high switching precision is possible. Transverse to the direction of movement of the belt (F), the light beam of this measurement site (5) has a substantially greater extension, for example, several millimeters. Thus, optical integration takes place over irregularities of the edge of the object to be detected (2), which also ensures the switching precision with, for example, fibers protruding from the edge of the object. The measurement site (5) detects not only the edge of the object (2) with a high degree of precision, but also detects scratches, markings, and adhesive seams of the transporting belt (1), transverse to the direction of movement of the belt (F), which would thus trigger a faulty switching of the outlet for the detection of the object. Therefore, these features are to be identified by means of at least one other measurement site (6,7) with suitable light beam geometry and are to be separated by an evaluation unit (8), from the actual process that is to be detectedxe2x80x94the entry of the edge of the object into the detection range. This purpose is met by at least one other measurement site (7), whose used light beam cross section has such a large area that it is not scratches, markings, adhesive, and connecting seams in this light beam that lead to a switching process at this measurement site (7), but rather the actual object (2). This measurement site (7) is advantageously located in the direction of movement of the belt before the measurement site (5) so that it is completely covered by the object (2), when its edge reaches the measurement site (5). Thus, the edge of the object (2) can be distinguished with sufficient clarity from the other features of the transporting belt (1) and its position can be precisely detected.
A prominent advantage of the apparatus in accordance with the invention is to be found in the very high precision of the switching point for the detection of the edge of the sheet with the guarantee that the edge of the sheet can be reliably distinguished from other features of the transporting belt (1) and thus false alarms due to these other features of the transporting belt (1) can be ruled out.